The ZBackingFile code was designed to allow platforms to decide if they want to use heap multi-mapping or some other (possibly HW supported) scheme. As of today, all our supported platforms do heap multi-mapping, so there's some degree of code duplication in ZBackingFile for each platform. I propose that we move this common code into ZPhysicalMemoryManager to remove this code duplication. If we in the future find that we want to support a platform that doesn't do multi-mapping, then we can introduce an abstraction for this again.